Fix: sessiond: fs.protected_regular sysctl breaks app registration

[lttng-tools.git] / src / bin / lttng-sessiond / shm.c

diff --git a/src/bin/lttng-sessiond/shm.c b/src/bin/lttng-sessiond/shm.c
index 23bd9db2085221f341454bee136c76b1a6b91c68..60a92cd15d3461850e898cdb95c4b00448ef12ba 100644 (file)
--- a/src/bin/lttng-sessiond/shm.c
+++ b/src/bin/lttng-sessiond/shm.c
@@ -70,11 +70,44 @@ static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
        /*
         * Try creating shm (or get rw access). We don't do an exclusive open,
         * because we allow other processes to create+ftruncate it concurrently.
+        *
+        * A sysctl, fs.protected_regular may prevent the session daemon from
+        * opening a previously created shm when the O_CREAT flag is provided.
+        * Systemd enables this ABI-breaking change by default since v241.
+        *
+        * First, attempt to use the create-or-open semantic that is
+        * desired here. If this fails with EACCES, work around this broken
+        * behaviour and attempt to open the shm without the O_CREAT flag.
+        *
+        * The two attempts are made in this order since applications are
+        * expected to race with the session daemon to create this shm.
+        * Attempting an shm_open() without the O_CREAT flag first could fail
+        * because the file doesn't exist. It could then be created by an
+        * application, which would cause a second try with the O_CREAT flag to
+        * fail with EACCES.
+        *
+        * Note that this introduces a new failure mode where a user could
+        * launch an application (creating the shm) and unlink the shm while
+        * the session daemon is launching, causing the second attempt
+        * to fail. This is not recovered-from as unlinking the shm will
+        * prevent userspace tracing from succeeding anyhow: the sessiond would
+        * use a now-unlinked shm, while the next application would create
+        * a new named shm.
         */
        wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
        if (wait_shm_fd < 0) {
-               PERROR("Failed to open wait shm at %s", shm_path);
-               goto error;
+               if (errno == EACCES) {
+                       /* Work around sysctl fs.protected_regular. */
+                       DBG("shm_open of %s returned EACCES, this may be caused "
+                                       "by the fs.protected_regular sysctl. "
+                                       "Attempting to open the shm without "
+                                       "creating it.", shm_path);
+                       wait_shm_fd = shm_open(shm_path, O_RDWR, mode);
+               }
+               if (wait_shm_fd < 0) {
+                       PERROR("Failed to open wait shm at %s", shm_path);
+                       goto error;
+               }
        }
 
        ret = ftruncate(wait_shm_fd, mmap_size);